Printed circuit boards and the methods of their production

ABSTRACT

It is an object of the present invention to provide a copper electroplating method for a printed circuit board having via-holes, which can reproducibly secure good plated film quality and via-hole filling capacity even when the board includes a resist or the like. The electroplating bath for electroplating of a printed circuit board, containing at least one compound selected from the group consisting of pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety.

FIELD OF THE INVENTION

[0001] The present invention relates to a printed circuit board in whicha circuit board is subjected to a copper electroplating to form adesired electrical circuit, more particularly to a printed circuit boardin which via-holes of a printed circuit board having blind via-holes arefilled with a copper electroplating, and a method for producing thesame. Further, the present invention relates to a copper electroplatingmethod for electroplating via-holes of a printed circuit board havingblind via-holes with copper, and a copper electroplating bath forelectroplating a varying board with copper.

BACKGROUND OF THE INVENTION

[0002] Recently, electronic devices are increasingly required to becompacter and more functional. The printed circuit board on which avarying electronic device is mounted is increasingly required to have adenser wiring, higher degree of integration and higher contactreliability, in order to cope with the above trends. In order to satisfythese requirements, the so-called build-up process, in which conductivelayer and insulating layer are alternately laminated on a base substrateafter undergoing a full-additive process or semi-additive process toform a fine and three-dimensional circuit, has been widely employed.

[0003] One example of the methods widely used for forming these finewirings comprises deposition of a thin copper film on a surface of aninsulating resin layer by sputtering or the like, forming a desiredresist pattern on the thin film and copper electroplating the laminate.It is a common practice in this method to secure an electrical contactbetween a conductive layer present below an insulating layer and aconductive layer provided above it by means of connecting holes by thename of via-holes, where each via-hole is plated with a conductivematerial on the wall or totally filled with a conductive material.

[0004] More specifically, an insulating layer is irradiated with carbondioxide gas or bored by a mechanical means (e.g., drill) on theconnecting or conducting passage area, and each hole is plated with aconductive material on the wall or totally filled with a conductivematerial. This procedure can secure electrical contact between theconducting layers with the insulating layer in-between, and is widelyemployed by the industry concerned.

SUMMARY OF THE INVENTION

[0005] However, it is more preferable to fill each via-hole totally witha conductive material by electroplating than to plate the via-hole wallonly with a conductive material, because of much improved reliability ofthe electrical contact.

[0006] Filling via-holes by electroplating carries another advantagewhen 3 or more conductive layers are laminated via an insulating layer,because the via-holes between the first and second conductive layers canbe arranged immediately above those between the second and thirdconductive layers.

[0007] In result, this structure reduces a space for the via-holes as awhole, thereby allowing the devices to be mounted at a higher density.Via-holes generally have a diameter of several tens microns inconsideration of the trends to finer wirings.

[0008] When via-holes are filled with a conductive material byelectroplating, it is necessary to form a thin film of conductivematerial beforehand by electroless copper plating or the like on thesurface to be electroplated. At the same time, special considerationsare required for the subsequent electroplating by which via-holes arefilled with copper.

[0009] More specifically, the surface, on which a conductive circuit isformed, has a flat surface other than via-holes and a concave portion ofvia-hole, with the result that it may be excessively coated with aplated conductive film when the via-holes are completely filled with aconductive material by electroplating. Conversely, when the surface isto be coated with a conductive layer of adequate thickness byelectroplating, the via-holes may be filled insufficiently.

[0010] In order to avoid these difficulties, copper electroplating bathsincorporated with several types of additives have been widely used tofill the via-holes with a conductive material by electroplating and, atthe same time, to coat the surface with a conductive layer of adequatethickness by copper electroplating.

[0011] The copper electroplating bath prepared to adequately fillvia-holes is incorporated with three types of additives (polymercomponent, leveler component and brightener component). The procedurefor filling via-holes with the above bath has been widely recognized.

[0012] Of these three types of additives, the leveler component isnormally of an organic dye. The mechanisms by which these three additivecomponents work for filling via-holes by copper electroplating are notfully understood. However, it is widely accepted that the levelercomponent as one of the three additive components works to controlcopper separation rate both inside and outside of each via-hole. This isdescribed below.

[0013] That is, the molecules constituting the leveler component areadsorbed on the boar surface to be electroplated to retard copperelectroplating, but are depleted on the electrode (board surface to beelectroplated) by being electrochemically decomposed or included in theplated film.

[0014] This depletion is accompanied by the leveler component moleculesdiffusing towards the board surfaces from the bulk bath, whoseconcentration remains essentially constant. Diffusion rate of theleveler component molecules is one of the factors that determine platingrate. The substance used as the leveler component generally has arelatively high molecular weight, and produces a gradient of itsconcentration in the passage (diffusion layer) between the bulk platingbath to the board surface at a plating rate beyond a certain level. Onthe other hand, the concave portion of the via-hole requires a longerdiffusion path than the flat portion free of the via-holes to have asmaller quantity of the diffused leveler component at the bottom thanthe upper flat surface free of the via-holes. Therefore, the levelercomponent is present always at a lower concentration on the via-holebottom surface than on the flat surface, with the result that the platedcopper film grows faster on the via-hole bottom surface than on the flatsurface. It is understood that the via-hole is filled with copper byelectroplating by the above effect.

[0015] The leveler component having the above characteristics may causeundesirable effect of inhomogeneity of plated film thickness, when it isunevenly distributed on the board surface.

[0016] The uneven concentration distribution of the leveler component ismore noted in the areas where it diffuses unevenly, e.g., in thevicinity of the resist wall, or the plating bath tends to flow unevenly,with the above-described via-hole insides and outsides set aside.

[0017] Therefore, electroplating to fill via-holes needs a plating bathcompletely free of leveler component or containing it at a lowconcentration. It is an object of the present invention to provide aplating method which fills via-holes while substantially removing theproblems caused by a leveler component and, at the same time, secures agood plated film of uniform thickness on the other portion.

[0018] An organic dye used as the leveler is generally expensive, anduse of a plating bath completely free of leveler component or containingit at a low concentration brings another advantage of reducing cost ofthe copper electroplating to fill the via-holes.

[0019] The problems resulting from use of a leveler are described takinga printed circuit board as an example, the similar problems should occurwith a semiconductor board which are provided with finer via-holes.

[0020] Therefore, it is another object of the present invention toprovide a printed circuit board electroplated with copper on the circuitboard to form a desired electrical circuit thereon, wherein thevia-holes in the printed circuit board are filled with copper byelectroplating. It is still another object of the present invention toprovide a copper electroplating method for a printed circuit board withblind via-holes to electroplate the via-holes. It is still anotherobject of the present invention to provide a copper electroplating bathfor electroplating a varying board with copper.

[0021] The inventors of the present invention have concluded, afterhaving extensively studied to solve the above problems, that it isnecessary to find out a method for filling via-holes by electroplatingwithout depending on a leveler component.

[0022] They have also extensively studied functions of each of theabove-described additives in searching for plating baths completely freeof leveler component or containing it in trace quantities, to find thata leveler component has two functions.

[0023] One of the functions is to form assemblies of the molecules inthe interface by stacking them, a property which many organic dyeexhibit. Formation of these assemblies tends to occur in defects, e.g.,dislocation or steps, in which addition of the copper atoms progressesmost notably. As a result, it retards progress of plating in thecathodic interface.

[0024] The other function, which a leveler component exhibits, is tobreak an adsorption film of polymer covering the electrode surface,which is coated and blocked with the polymer component, e.g.,polyethylene glycol, thereby facilitating access of the copper ion orbrightener component to the board surface. It is a property of organic,ionic compounds on which leveler component charges are highlydelocalized.

[0025] In other words, a leveler component represented by an organic dyeis found to have two functions contradictory to each other, one isretarding the plating reaction and the other accelerating the reaction.For the latter function, a leveler is not necessarily of an associativedye. At the same time, it is meant that even an organic, ionic compoundof relatively low molecular weight can exhibit the function, whenfilling of via-holes is achieved mainly by a brightener componentfunction.

[0026] When a brightener approaches the electrode surface, the surfacearea in the via-hole is reduced as the plating reaction proceeds, withthe result that the brightener component is concentrated on the surface,and the reaction proceeds faster in the via-hole inside than in theoutside. The via-hole filling process by the brightener concentration onthe surface is based on its property of being depleted at a lower ratein the plating reaction process.

[0027] Based on the above concept, the inventors of the presentinvention have searched for alternative via-hole filling additiveshaving a lower molecular weight than the conventional leveler component,not associative at least at around a plating potential, soluble in waterand active in the interface.

[0028] The associative property of the conventional leveler component isclosely related to the fact that it is of an organic dye. Therefore, itis an important condition for the additive component that replaces theconventional leveler component not to substantially exhibit strong lightabsorption in the visible region.

[0029] The new leveler component will exhibit a high via-hole fillingcapacity when used in combination with the brightener component, asdiscussed above. It is found that the new additive for the presentinvention can fill the via-holes and, at the same time, give the platedfilm of high homogeneity on the portion other than the via-holes.

[0030] It is also found, in the case of a board with a plated resistpattern, that the new additive can give a highly flat and excellentplated film even in the vicinity of the wall. It is well known that theconventional leveler frequently deteriorates plated film flatnesssignificantly in the vicinity of the plated resist wall.

[0031] The good results demonstrated by the method of the presentinvention results from its independence on a compound of very lowdiffusibility used for the conventional leveler for via-hole filling.

[0032] In the method of the present invention, which uses no componentof low diffusibility, problems resulting from diffusion of the copperion itself, which are concealed in the presence of the leveler componentof strong functions, tend to be more noted.

[0033] These problems sometimes observed in the vicinity of the platedresist wall. They result from retarded diffusion of the copper ionacross the resist wall. The effect of retarded symmetry of the copperion diffusion is much smaller than that of a leveler, but exactlyopposite in direction.

[0034] In other words, the copper ion is more used for plating as itdiffuses more, unlike the case with a leveler. It is possible to improvehomogeneity of the plated film thickness to a still higher extent byutilizing the functions of the cooper ion and leveler component workingin the opposite direction under the same geometric conditions whichlimit diffusion of the solute.

[0035] The above can be achieved by only incorporating a trace quantityof a leveler component in the electroplating bath, in addition to theabove-described polymer component, brightener component and organicionic compound. A leveler is incorporated only in a quantity sufficientfor canceling out small fluctuations of film thickness resulting fromasymmetry of the copper ion diffusion.

[0036] The inventors of the present invention have noted a specificquaternary, nitrogen-containing heterocyclic compound as the copperplating promoter which satisfies the above requirements, achieving thepresent invention.

[0037] Thus, the first aspect of the present invention relates to aprinted circuit board, the second aspect relates to a method forproducing the same, the third aspect relates to an electroplatingmethod, and the fourth aspect relates to a copper electroplating bath.The copper electroplating bath is incorporated with at least onecompound selected from the group consisting of pyridinium, bipyridinium,phenanthrolinium, quinolinium and phenazinium salts in the form of oniumwith an N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety asa copper plating promoter.

[0038] In the present invention, the pyridinium, bipyridinium,phenanthrolinium, quinolinium and phenazinium salts in the form of oniumwith an N-alkyl, N-aralkyl or N-aryl moiety etc. mean an organicpyridinium cation represented by the general formula:

[0039] (wherein, R₁ is an organic moiety selected from the groupconsisting of alkyl, phenyl and aralkyl moiety), an organic bipyridiniumcation represented by the general formula:

[0040] (wherein, R₂ and R₃ are each an organic moiety selected from thegroup consisting of alkyl, phenyl and aralkyl moiety),

[0041] an organic phenanthrolinium cation represented by the generalformula:

[0042] (wherein, R₄ is an organic moiety selected from the groupconsisting of alkylene and aralkylene moiety), an organic quinoliniumcation represented by the general formula:

[0043] (wherein, R₅ is an organic moiety selected from the groupconsisting of alkyl, phenyl and aralkyl moiety), and

[0044] an organic phenazinium cation represented by the general formula:

[0045] (wherein, R₆ is an organic moiety selected from the groupconsisting of alkyl, phenyl and aralkyl moiety),

[0046] Of these five types of the cationic salts, the organicbipyridinium salt is particularly preferable for its excellent via-holefilling capacity and availability in the markets.

[0047] The N-alkyl, N-phenyl and N-aralkyl moieties etc. represented byone of R₁ to R₅ may be substituted with a varying substituent orunsubstituted. Sulfonic acid moiety is a preferable substituent, becausethese moieties substituted therewith can be more compatible with thesulfonic acid anion normally present in the copper electroplating bath.

[0048] The pyridinium, bipyridinium, phenanthrolinium, quinolinium andphenazinium salts in the form of onium with an N-alkyl, N-aralkyl orN-aryl moiety etc. are clearly distinguished from an organic dye whichhas been traditionally used as the leveler component. These cationicsalts are characterized by a low molar extinction coefficient of only5000 M⁻¹cm⁻¹ (M: moles/litter) in a wavelength region of 400 to 700 nm.

[0049] Other objects, features and advantages of the invention willbecome apparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050]FIG. 1 is a cross-sectional view of the printed circuit boardelectroplated with copper, prepared in EXAMPLE 1, for the portioncontaining via-holes, drawn based on the microgram.

[0051]FIG. 2 is a cross-sectional view of the printed circuit boardelectroplated with copper, prepared in COMPARATIVE EXAMPLE 1, for theportion containing via-holes, drawn based on the microgram.

[0052]FIG. 3 is a cross-sectional view of the printed circuit boardelectroplated with copper, prepared in COMPARATIVE EXAMPLE 5, for theportion containing via-holes, drawn based on the microgram.

DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS

[0053]1: Base substrate

[0054]2: Copper foil layer

[0055]3: Resist

[0056]4: Epoxy resin layer

[0057]5: Copper-plated film

DETAILED DESCRIPTION OF THE INVENTION

[0058] The via-holes for the present invention are generally 10 to 600μm deep and 5 to 800 μm in diameter for printed circuit boards. Thesevia-holes can be produced by boring an insulating resin layer putbetween two conductive layers with laser beams. It is needless to saythat the via-holes are finer when the copper plating of the presentinvention is applied to a semi-conducting board.

[0059] Each via-hole can be electroplated, when its bottom and sides aremade conductive beforehand by electroless plating or sputtering. It isalso possible to make a desired portion outside of the via-holeconductive by a similar means.

[0060] The representative composition of a copper electroplating bathfor filling via-holes by the copper electroplating method of the presentinvention is copper sulfate pentahydrate: 55 to 240 g/L, concentratedsulfuric acid: 60 to 260 g/L and chlorine ion content: 60 to 100 ppm. Itshould be noted that implementation of the present invention does notbecome immediately difficult when the composition is out of the aboverange.

[0061] The base composition prepared above for the copper electroplatingbath is incorporated with polyethylene glycol as a polymer component at20 to 300 mg/L and sodium salt of bis-(3-sulfopropyl) disulfide(hereinafter referred to as SPS), represented by (NaO₃S—CH₂—CH₂—CH₂—S—)₂, as a brightener component at 1 to 30 mg/L. The above solution isfurther incorporated with at least one selected from the groupconsisting of the above-described five types of cationic salts, i.e.,pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenaziniumsalts in the form of onium with an N-alkyl, N-aralkyl or N-aryl moietyetc. at 2 to 2000 μmols/L for the electroplating.

[0062] It is preferable, depending on circumstances, to incorporate atrace quantity of an organic dye or the like as a leveler in theelectroplating bath composition prepared above to still improve flatnessof the plated film. The anions which can be suitably used as the counterion of the above-described organic cation include a halide ion, e.g.,chloride ion, and sulfonate ion. The ionic compound is incorporated asthe counter ion at a much lower content than any other component of thebase plating bath, and cannot greatly affect the ionic concentration ofthe bath. No trouble is anticipated when the chloride or sulfonate ionoriginally present in the base composition of the plating bath is usedas the counter ion, unless its concentration is out of the originallyintended suitable ion concentration range.

[0063] For energizing the cathode and anode for electroplating, either apotentiostatic or galvanostatic method can satisfy the object of thepresent invention. Galvanostatic copper electroplating is preferablycarried out at a current density applied of 2.5A dm⁻², but the effect ofthe present invention can be realized at 0.5 to 6.0A dm⁻².

EXAMPLES

[0064] The present invention is described by EXAMPLES.

Examples 1

[0065] A base substrate having a copper foil coated on a surface thereofwas coated with a 15 μm thick insulating layer of known epoxy resinnormally used for the build-up process, and then the surface wasirradiated with carbon dioxide gas laser beams to form via-holes, 60 μmin diameter at the opening, 50 μm in diameter at the bottom and 15 μmdeep. The coated base substrate was immersed in a permanganic acidsolution for desmearing, and then provided with a catalyst by a knowntreatment method, to form a 0.6 μm thick electroless, copper-platedfilm. The treated base substrate was cut into 60 mm by 60 mm specimensin such a way that each had via-holes. Each specimen was electroplatedat a current density of 2.5A dm⁻² for 26 minutes in a plating bath ofthe composition given in Table 1 while it was kept upright. During theplating, the bath was stirred by airing in the electroplating tankhaving inner dimensions of width: 250 mm, depth: 200 mm and height: 200mm.

[0066] The electroplated base substrate having via-holes was cut andground, to observe the via-hole cross-section. FIG. 1 presents amicroscopic cross-sectional view which schematically shows the basesubstrate electroplated with copper, prepared in EXAMPLE 1. As shown,the via-hole was filled with copper by electroplating. The surface wasgood in flatness, with irregularities of only around ±2 μm even in thevicinity of the resist. TABLE 1 Composition of the plating bath used inEXAMPLE 1 Plating bath components Concentration Copper sulfatepentahydrate 200 g/L Concentrated sulfuric acid  60 g/L Hydrochloricacid (0.1 mol/L)  17 mL/L Polyethylene glycol 4000 100 mg/L SPS  5 mg/LBenzyl biologen chloride  25 μmol/L

Comparative Example 1

[0067] A base substrate was electroplated in the same manner as inEXAMPLE 1, except that the electroplating bath composition given inTable 1 contained no benzyl biologen chloride. The electroplated basesubstrate having via-holes was cut and ground to observe the via-holecross-section. FIG. 2 presents a microscopic cross-sectional view whichschematically shows the base substrate electroplated with copper,prepared in COMPARATIVE EXAMPLE 1. As shown, the via-hole was not filledsufficiently with copper by electroplating. The plated film surface wasfound to have irregularities of ±5 μm or more.

Example 2

[0068] A base substrate was electroplated in the same manner as inEXAMPLE 1, except that the plating bath composition was replaced by theone given in Table 2. The electroplated base substrate having via-holeswas cut and ground to microscopically observe the via-holecross-section. It was observed that the via-hole was filled with copperby electroplating, and the copper-plated film surface was good inflatness in the vicinity of the resist. TABLE 2 Composition of theplating bath used in EXAMPLE 2 Plating bath components ConcentrationCopper sulfate pentahydrate 200 g/L Concentrated sulfuric acid  80 g/LHydrochloric acid (0.1 mol/L)  17 mL/L Polyethylene glycol 4000 200 mg/LSPS  8 mg/L Hexylpyridinium chloride  35 μmol/L

Compapative Example 2

[0069] A base substrate was electroplated in the same manner as inEXAMPLE 2, except that the electroplating bath composition given inTable 2 contained no hexylpyridinium chloride. The electroplated basesubstrate having via-holes was cut and ground to microscopically observethe via-hole cross-section. It was observed that the via-hole was notfilled sufficiently with copper by electroplating, and the plated filmsurface had irregularities of ±5 μm or more.

Example 3

[0070] A base substrate was electroplated in the same manner as inEXAMPLE 1, except that the plating bath composition was replaced by theone given in Table 3. The electroplated base substrate having via-holeswas cut and ground to microscopically observe the via-holecross-section. It was observed that the via-hole was filled with copperby electroplating, and the copper-plated film surface was high inflatness showing no phenomenon of growth of film thickness in thevicinity of the resist. TABLE 3 Composition of the plating bath used inEXAMPLE 3 Plating bath components Concentration Copper sulfatepentahydrate 200 g/L Concentrated sulfuric acid 100 g/L Hydrochloricacid (0.1 mol/L)  17 mL/L Polyethylene glycol 4000 100 mg/L SPS  6 mg/LHexylquinolinium chloride  30 μmol/L

Compapative Example 3

[0071] A base substrate was electroplated in the same manner as inEXAMPLE 3, except that the electroplating bath composition given inTable 3 contained no hexylquinolinium chloride. The electroplated basesubstrate having via-holes was cut and ground to microscopically observethe via-hole cross-section. It was observed that the via-hole was notfilled sufficiently with copper by electroplating, and the plated filmsurface had irregularities of ±5 μm or more.

Example 4

[0072] A base substrate was electroplated in the same manner as inEXAMPLE 1, except that the plating bath composition was replaced by theone given in Table 4. The electroplated base substrate having via-holeswas cut and ground to microscopically observe the via-holecross-section. It was observed that the via-hole was filled with copperby electroplating, and the copper-plated film surface was high inflatness showing no phenomenon of growth of film thickness in thevicinity of the resist. TABLE 4 Composition of the plating bath used inEXAMPLE 4 Plating bath components Concentration Copper sulfatepentahydrate 200 g/L Concentrated sulfuric acid  70 g/L Hydrochloricacid (0.1 mol/L)  17 mL/L Polyethylene glycol 4000 150 mg/L SPS  7 mg/LEthylphenazinium chloride  30 μmol/L

Comparative Example 4

[0073] A base substrate was electroplated in the same manner as inEXAMPLE 4, except that the electroplating bath composition given inTable 4 contained no ethylphenazinium chloride. The electroplated basesubstrate having via-holes was cut and ground to microscopically observethe via-hole cross-section and the via-hole periphery in contact withthe resist wall. It was observed that the via-hole was not filledsufficiently with copper by electroplating, and the plated film surfacehad irregularities of ±5 μm or more.

Comparative Example 5

[0074] A base substrate was electroplated in the same manner as inEXAMPLE 1, except that the electroplating bath composition containedJanus Green B as a leveler in place of benzyl biologen chloride for thepresent invention (Table 5). The electroplated base substrate havingvia-holes was cut and ground to microscopically observe the via-holecross-section and the via-hole periphery in contact with the resistwall. FIG. 3 presents a microscopic cross-sectional view whichschematically shows the base substrate electroplated with copper,prepared in COMPARATIVE EXAMPLE 5. As shown, the via-hole was filledwith copper to some extent by the electroplating, but thickness of theplated film increased to 5 μm or more in the area in contact with theresist wall. It was therefore found that flatness of the plated film asone of the objects of the present invention could not be secured. TABLE5 Composition of the plating bath used in COMPARATIVE EXAMPLE 5 Platingbath components Concentration Copper sulfate pentahydrate 210 g/LConcentrated sulfuric acid  80 g/L Hydrochloric acid (0.1 mol/L)  17mL/L Polyethylene glycol 4000 120 mg/L SPS  5 mg/L Janus Green B  20μmol/L

[0075] It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

Effects of the Present Invention

[0076] As described above, the method of the present invention forelectroplating a board with via-holes can well fill the holes withcopper and realize high flatness of the plated film.

What is claimed is:
 1. A printed circuit board having via-holes platedby a copper electroplating, wherein the via-holes are substantiallyfilled with a copper electroplating using a copper electroplating bathcontaining at least one compound selected from the group consisting ofpyridinium, bipyridinium, phenanthrolinium, quinolinium and phenaziniumsalts in the form of onium with an N-alkyl, N-aralkyl, N-aryl,N-alkylene or N-aralkylene moiety.
 2. A method for producing a printedcircuit board having via-holes plated by a copper electroplating,wherein an electroplating bath used in the copper electroplatingcontains at least one compound selected from the group consisting ofpyridinium, bipyridinium, phenanthrolinium, quinolinium and phenaziniumsalts in the form of onium with an N-alkyl, N-aralkyl, N-aryl,N-alkylene or N-aralkylene moiety.
 3. An electroplating method whichconducts a copper electroplating using an electroplating bath containingat least one compound selected from the group consisting of pyridinium,bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in theform of onium with an N-alkyl, N-aralkyl, N-aryl, N-alkylene orN-aralkylene moiety.
 4. The electroplating method according to claim 3,wherein the N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylenemoiety contains sulfonic acid moiety.
 5. The electroplating methodaccording to claim 3, wherein the salts have a low molar extinctioncoefficient of 5000 M⁻¹cm⁻¹ (M: moles/litter) in a wavelength region of400 to 700 nm.
 6. A copper electroplating bath used in a copperelectroplating which contains at least one compound selected from thegroup consisting of pyridinium, bipyridinium, phenanthrolinium,quinolinium and phenazinium salts in the form of onium with an N-alkyl,N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety.
 7. The copperelectroplating bath according to claim 6, wherein the N-alkyl,N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety contains sulfonicacid moiety.
 8. The copper electroplating bath according to claim 6,wherein the salts have a low molar extinction coefficient of 5000M⁻¹cm⁻¹ (M: moles/litter) in a wavelength region of 400 to 700 nm.